Method and apparatus for connecting a getter-ion pump to an analytical mass spectrometer



Aug 21, 1962 E w. BOYER ET AL 3,050,622

METHOD AND APPARATUS FOR CONNECTING A GETTER-ION PUMP TO AN ANALYTICAL.MASS SPECTROMETER Filed 0G11. 3, 1960 vAcuuM 2a 21 26 souRcE IN V ENTORS ERNEST W. BOYER HARRELL 72 FORD ERNEST E. MKELVEY ATTORNEY UnitedStates Patent O 3,050,622 METHOD AND APPARATUS FOR CONNECTING AGETTER-ION PUMP T AN ANALYTCAL MASS SPECTROMETER Ernest W. Boyer,Harrell T. Ford, and Ernest E. McKelvey, Ponca City, Okla., assignors toContinental Oil Company, Ponca City, Okla., a corporation of Dela- WareFiled Uct. 3, 1960, Ser. No. 59,870 6 Claims. (Cl. Z50-41.9)

This invention relates generally to lthe adaptation of an ion vacuumpump to an analytical mass spectrometer and in particular to thehardware used to connect the ion vacuum pump to the analytical massspectrometer.

This application rerers -to the novel construction of hardware forconnecting an ion pump to an analytical mass spectrometer. The methodfor operatably connecting an ion pump to an analytical mass spectrometeriS described in greater detail in an application entitled Method andApparatus for Operating An Analytical Mass Spectrometer with aGetter-Ion Pump, by Ernest W. Boyer, iiled concurrently with thisapplication and assigned to the same assignee. I

Since the development of the analytical lmass spectrometer the diffusionpump has been the source for obtaining the high vacuum required for itsproper operation. While the diffusion pump has been more than adequatein obtaining a satisfactory vacuum for the proper performance of thespectrometer, the industry has been plagued with the length of time thatthe pump requires to obtain an adequate vacuum, especially withcondensable light hydrocarbon. Maximum or minimum nitrogen levels in thecold traps fail to solve the problem. Even when a programmed nitrogenlevel was used, the solution was far from satisfactory. The problem isfurther compounded lby a compound such as ethane which is condensed to avolatile liquid under trap conditions and is in equilibrium with coldtrap and source region. Under these conditions, it causes a variablebackground which can not be eliminated by simply subtracting thebackground as measured before recording a sample spectrum.

The ion pump, on the other hand, permits the elimination of thetroublesome cold trap and thus permits direct coupling between the ionpump and source. The application of an ion pump to a mass spectrometeris not new; however, the use was limited solely to a spectrometer whichwas analyzing simple systems. Since the ion pump is species selective,that is, will pump one entity faster than another, it was determined toybe impossible to apply the pump to an analytical mass spectrometer thatobviously requires the pumping of widely dierent compounds. Themolecules having different sizes and atomic structures would be operatedon by the electronic stream of the pump differently. The pumping ratewould, of course, be determined by not only the molecular size, that is,how large an area the molecule presented to the electronic stream, butalso the electronic coniiguration. However, the ion pump creates newproblems since it is species selective in its pumping rate. Thus, theion pump will pump various entities at a different rate, therebyupsetting the ratio of the entities at the source and resulting inerrors in the analysis of the mixtures under test. The ion pump has afurther ditliculty in that if the entire system requires venting for anyreason, such as maintenance on the source, the system must be opened tothe atmosphere. When 4the system is again evacuated, the ion pump isroughed to approximately 20 microns; when this pressure is reached, thepower supply to the ion pump is energized. The nature of the pump issuch that at operation at these pressures (about 5 microns) the pumpheats up. At elevated temperatures, for some reason not completelyunderstood, chemical entities previously ICC pumped are released fromsurfaces within the pump. These chemical entities so released canmigrate to and contaminate the spectrometer source. rllhese depositedentities will result in erroneous analysis of samples under test.

Entities, as used in this specication, shall mean any particles orcombination of particles (molecules, atoms, etc.) in a charged oruncharged state.

It is, therefore, an object of this invention to disclose apparatus forconnecting an ion vacuum pump to an analytical mass spectrometer.

It is a further object of this invention to provide unique apparatusthat will isolate the mass spectrometer from the electronic pump `duringthe initial start-up of Said pump.

It is a still further object of this invention to prevent contaminationof the analytical mass spectrometer source from material degassed oremitted from the pump during the start-up cycle.

It is another object of this invention to disclose a method ofconnecting the ion pump so that it will never become vented during theroutine maintenance or use of the equipment, thus prolonging the life ofthe pump.

It is a further object of this invention to reduce pumpdown timerequired for the `analytical mass spectrometer after venting of thesource.

yIt is a still further object of this invention to provide apparatusthat will permit connection of an electronic pump to the usualanalytical mass spectrometer in service today and further preventinterference of the ion pump magnet with th magnetic field of theanalytical mass spectrometer.

This invention features hardware that is particularly adapted to connectan ion pump to an analytical mass spectrometer source and comprises arst pipe which has a flange on one end which is adapted to mate with theion pump output and an isolation valve on the other end which is adaptedto isolate the ion pump when the source is purged. A second pipeconnects the isolation valve with the mass spectrometer source.Connected to both the first and second pipes and communicating therewithare two valves which may be jointly or selectively connected to amechanical vacuum pump.` When the source must be purged, the isolationvalve is closed; the valve in the second pipe is opened and connected tothe mechanical pump thereby permitting evacuation of the pipe prior toreopening the isolation valve. The valve connected to the first pipe,when connected to the me chanical pump, provides a method for evacuatingthe pipe prior to the operation of the ion pump.

The invention further features a method for eliminating secondaryemission originating in the pump from migrating to the source bymounting the rst and second pipes at right angles, thus the right anglebend tends to ground secondary emission particles leaving the ion pump,thereby preventing said secondary emission particles from entering thesource and upsetting sample portions under test.

'Ihis invention also `features bailles added to the line near the pumpto aid in minimizing secondary emission from the pump after evacuationhas occurred.

Further objects, features, and advantages of the invention will becomeapparent from the following description and claims when read in View ofthe accompanying drawing which is a partial sectional drawing of thehardware used tot couple the ion pump to the spectrometer source.

Referring to the drawing, an ion pump 1t) is shown connected to ananalytical mass spectrometer source 111 through air-tight hardware whichcomprises a cylindrical tube 12 which has a flange 13 adapted to matewith an output flange 14 of the ion pump. Flanges 13 and 14 are heldtogether by any suitable means such as a plurality of bolts 15. A highvacuum valve 16 is connected to the end of cylindrical tube 12 oppositeange 13 by any suitable means such as shoulder 117. High Vacuum valve 16may be any suitable valve capable of maintaining an air-tight sealduring operation in either a closed or open position. The type of valve16 used is commonly available and briefly incorporates a valve seat 18,a Valve head 19 adapted to seal the end of cylinder `12 when the valvehead and seat are mated. A threaded valve stem 20 passes through valveend 21 and has one end rotatably attached to valve head 19 and the otherend rigidly attached to a suitable handle or nob 22. In order tomaintain an adequate seal at all times between the valve end 21 andvalve head 19, a bellows 23 is sealably attached to the top of the valveand tothe periphery of the valve head. Thus, any air escaping into theinner portion of the valve around the valve stem threads is confinedwithin the bellows and does not enter the inside of cylindrical tube 12.

The output 24 of valve \16 is connected to a cylindrical portion 25.Portion 25 includes a pair of pipes 26 and 27, respectively, attached toits periphery. The irst input 26 is connected to a vacuum gaugemeasuring means 2S yand the second input 27 is connected to a valve 29.The output of cylindrical portion 25 is connected through a cylindricalportion 30 to spectrometer source 11. For convenience, cylindricalportion 30 is here shown to contain a labyrinth comprising a pluralityof bafes 31 supported within cylindrical portion 30 by any suitablemeans. Also connected to cylindrical portion 12 is a second valve .32which is shown to be a valve similar to valve 16 and, therefore, willnot be described. The size of valve 32 permits rapid evacuation of thechamber within cylindrical tube 12; however, it is obvious that othertypes of valves may be used for either valve 32 or valve 16 providingthey are sufciently air-tight during their operation in either the openor closed position. Connected to the outlet of Valve 32 is `a pipe 33which is connected to the exhaust exit 34 of a mechanical pump 35. Apipe 36 is connected between the outlet of valve 29 and exhaust exit 34of mechanical pump 35. A valve 37 is connected between pipe 36 and thelair to permit venting of the spectrometer source in the event that workmust be done on the unit when the mechanical pump is connected.Mechanical pump 35 and pipe 36 may be disconnected by including adisconnect 33 and 39, thereby freeing the pump for other uses in thelaboratory. Annular rings 42 and diaphragm 43 are rigidly attachedWithin cylindrical tube 12 and aid in the elimination of secondaryemission entities.

Operation Referring to the drawing, |with Valve 16 in an open positionion pump 1h evacuates cylindrical tube y12, cylinder portions 25 and3l), and spectrometer source 1l; both valves 32 and 29 under normaloperation are operated in a yclosed position thereby preventing theentry of ai-r into the evacuated system. Inserted within tube portion 30is shown a plurality of bailles or diaphragms 31. These baffles show onemethod of increasing the source conductance so that the pump can notpump each species in the analytical spectrometer source at a diferentrate. Other methods of controlling the source conductance are possible,as for example, a plurality of right-angle bends. When for some reasonthe source must be shut down and the system vented, valve 16 is closed.This Will permit continuous operation of pump 1t), and will maintaincylindrical tube 12 in a highly evacuated state, thereby greatlyreducing the time required to pump-down the sys-tem when the Work on thesource is completed. If mechanical pump 35 is connected in the mannershown in the drawing, that is, pipe 36 is connected to valve 29, valve37 must be opened in order to let air into the system. Once the work iscompleted, valve 37 is closed and the mechanical pump energized.Operation of the mechanical pump will then partially evacuate thespectrometer source and cylinder portions 25 and 39, thus clearlyreducing the work required by the ion pump, and likewise, greatlyincreasing the speed that the system can be totally evacuated once theion pump is connected to a spectrometer source. When vacuum gauge 4l)indicates that the vacuum is as well as can be obtained by themechanical pump, valve 29 is closed and valve 16 opened. The remainingair in tube portions 25 and 30 and spectrometer source 1l1 will bebrought to operating vacuum. If for some reason the ion pump must bechanged or replaced thereby requiring that cylindrical tube 12 bepurged, rapid evacuation is obtained by connecting mechanical pump 35through pipe 33 to valve 32. When valve 32 is opened and mechanical pump35 actu-ated, the system is rapidly brought to a near operating rate bythe mechanical pump. When this occurs, valve 32 is closed and the ionpump energized and the system brought rapidly to its operating vaccum.

Valve 16, while operating as a valve, also presents a unique feature notapparent. When, for example, the analytical mass spectrometer has beenused to test samples of hydrocarbon and ion pump 10 has been used for aperiod of time, hydrocarbon molecules will become imbedded in thetitanium surface on the inside of ion pump 10. If for some reason, aspreviously explained, the ion pump is disconnected or cylindrical tube12 is filled with air, these molecules must then be evacuated when thepump is again re-energized. When the pump is initially started, theelevated temperatures Which develop cause a release of entitiespreviously pumped which may migrate up cylindrical tube 12 and back tosource 11. This emission or migratory molecules if permitted to reachsource 11 would obviously introduce errors during analysis. Furthersecondary emission entities released by the pump during normal operationwould enter the source and contaminate same if the pump and source werein line of sight with each other. This, however, is eliminated in ourunique construction by placing cylinder 25 at right angle to cylinder12. Thus, as secondary emission entities leave ion pump 10, they will beinhibited from entering cylinder 25 by ,the right-angle bend. Thus, asthe molecules travel up cylindrical tube 12, they will strike valve head19 causing them to become grounded or deflected and will migrate back topump 10. It is obvious Vthat if valve 16 is not included in the systemthat a baille or diaphragm or series of annular rings or bales could beinserted within cylindrical tube 12 and accomplish the same results.Further, cylindrical tube 12 may be bent at right angles and therebyresult in the grounding or deecting of the charged or unchargedentities, the end result being, of course, to prevent -the strayentities from migrating back to the spectrometer source and causing aninterference in analysis.

While annular ring 42 and diaphragm 4;'3 are shown in cylindricalportion 12, they may be eliminated if valve 16 provides suicientisolation for the sample under test. If valve 1 6 is eliminated, rings42 and diaphragm 43 would be necessary to eliminate the secondaryemission entities. Rings 42 and diaphragm 43 combine to form adellection means such that entities traveling from the ion pump backtoward the source will strike the diaphragm or rings and be grounded ordeflected from their straightline path resulting in their being drawnback to the pump rather than migrating on toward the source.

This invention has described in detail the unique hard- -ware used toconnect an ion pump to an analytical mass spectrometer; however, theparticular hardware can be readily adapted to other uses. For example,it provides an eiective method of connecting an ion pump to an electronmicroscope or to any mass spectrometer or timeof-flight mass spectromersince in all cases the unique problems presented by the ion pump arepresent.

Valve '16 will provide an easy method for preventing the ion pump frombeing exposed to the atmosphere, thereby extremely prolonging its life.Further, the special valves 29 and 32 provide an additional method ofextending the life of the ion pump by permitting evacuation of thesystem prior to operation of said pump.

While the preferred embodiments disclose circular tubing, it is obviousto one skilled in the art that other forms and shapes of tubing may beemployed without departing from the spirit and scope of this inventionand that the principle of operation of the apparatus regardless of itsgeneral physical appearance would represent the true invention.

Although this invention has been described with respect to particularembodiments thereof, it is not to be so limited, as changes andmodiiications may be made therein which are within the spirit and scopeof the invention as defined by the appended claims.

We claim:

1. Apparatus for retaining a vacuum on a mass spectrometer tube,comprising:

an ion pump;

pipe means connecting the ion pump to the spectrometer tube; and

electrically conductive obstruction means in the pipe means electricallygrounded to the spectrometer tube and positioned to preventline-of-sight communication between the ion pump and the spectrometertube, whereby charged entities tending to migrate from the ion pump tothe spectrometer tube are grounded by the obstruction means and aredrawn Iback to the ion pump 6 to prevent erroneous operation of thespectrometer tube.

2. Apparatus as defined in claim 1 wherein the obstruction meanscomprises a bend in the pipe means.

3. Apparatus as dened in claim 1 wherein the obstruction means comprisesa baille assembly.

4. Apparatus as defined in claim l wherein the obstruction meanscomprises a valve interposed in the pipe means.

5. Apparatus as defined in claim 4 characterized further to include amechanical pump, `and a second valve connecting the mechanical pump tothe pipe means between the rst mentioned valve and the spectrometertube, whereby the spectrometer tube and the pipe means between thespectrometer tube and the first mentioned valve may be evacuated by themechanical pump upon closure of the rst mentioned valve while continuingthe operation of the ion pump.

6. Apparatus as defined in claim 5 characterized further to include athird valve connecting the mechanical pump to the pipe means between theion pump and the first mentioned valve, whereby the mechanical pump maybe used to selectively evacuate the pipe means between the ion pump andthe first mentioned valve prior to startup of the ion pump.

References Cited in the file of this patent UNITED STATES PATENTS2,769,912 Lupfer et al. Nov. 6, 1956

